Use more RenderBox references in RenderFlexibleBox

RenderFlexibleBox has several methods that take pointers to RenderBox instances
but those methods assumes that the objects will never be NULL. We should replace
them by references. This makes the caller code safer.

This is in continuation to the CL https://codereview.chromium.org/555213002/

Review URL: https://codereview.chromium.org/560583003

git-svn-id: svn://svn.chromium.org/blink/trunk@181922 bbb929c8-8fbe-4397-9dbb-9b2b20218538